I love the classic stuff, too, but don't overlook some of the fantastic more recent works either. Robert B. Parker's (of Spencer fame) westerns starring Vigil Cole and Everett Hitch are excellent (
Appaloosa,
Resolution and
Brimstone). I can also recommend James D. Best's
Steve Dancy stories. The first book (
The Shopkeeper) is available for $4.99 - $5.99 pretty much anywhere ebooks are sold.
I'd also feel remiss if I didn't mention Larry McMurtry's
Lonesome Dove.
Blue Heaven by C.J. Box is a modern day thriller with a classic western subplot that might serve as a nice transition from crime/mystery to westerns.